Installations for manufacturing thermoplastic containers comprise, for heating the preforms prior to the blow-moulding or stretch-blow-moulding stage, heating installations for example of the tunnel oven type which traditionally are fitted with incandescent infrared radiation lamps.
Attempts have, however, been made to employ heating means of a different type such as heating using coherent infrared electromagnetic radiation of the laser radiation type. Such a radiation offers the advantage of being more directional and of allowing the bodies of the preforms to be heated far more precisely and in a more localized manner than can be achieved with traditional infrared radiation heating means the angular spread of which is relatively great; it also offers the advantage of more uniform absorption of the heat within the thickness of the wall of the preform.
In a first possible embodiment (for example FIGS. 6 and 7 of document FR 2 878 185), the or each source of coherent infrared electromagnetic radiation is directed substantially at right angles to the path followed by the bodies of the preforms. Such an arrangement is admittedly satisfactory as far as the heating of the bodies of the preforms is concerned, particularly with regard to the selectivity of this heating when such selectivity is desired, but also presents disadvantages.
One disadvantage with this known arrangement lies in the fact that the radiation passes through each body while heating the material thereof, but is not fully absorbed. The fraction of the radiation that has not been absorbed is reflected by a reflector positioned opposite the source and is returned towards the bodies of the preforms and towards the sources. However, this reflection is accompanied by partial absorption and by heating of the reflector, leading to a loss of energy. As a result, the efficiency of such a heating arrangement is not optimal.
What is more, a proportion of the reflected fraction of the radiation may return to the source, and this is detrimental to the life of this source.
Another disadvantage with this known arrangement lies in the fact that, when a space between the bodies of two consecutive preforms passes in front of the source of electromagnetic radiation, all of the radiation emitted thereby then reaches the reflector and, for the most part, is reflected back towards the source, with a risk of damaging, or even of destroying, the latter. Admittedly, it might be possible to contrive for the radiation to be emitted only for the time during which the body of a preform is moved in front of the source and for the emission of radiation to be interrupted when it is a space between two consecutive preform bodies that is moving in front of the source. Such a solution could be implemented, for example, by sequential excitation of the source which would then operate discontinuously, synchronized with the rate of travel of the preforms. This would, however, result in an installation that was complicated and expensive to produce and to keep in correct working order.
In another possible embodiment (for example FIGS. 9 and 10 of the same document FR 2 878 185), the or each source of coherent infrared electromagnetic radiation is directed substantially along the path followed by the bodies of the preforms so that the radiation passes in succession through a plurality of consecutive preform bodies. An arrangement such as this is admittedly satisfactory in terms of the actual heating of the preform bodies and the efficiency may be regarded to be better than that of the previous solution. However, this known arrangement has a disadvantage inherent to the fact that the path of the preform bodies has to be diverted away from the source just upstream thereof, or in other words, that the conveyor conveying the preforms needs to turn through a bend in front of the source.
Furthermore, the absorption of electromagnetic radiation varies with the material of which the preforms are made, and an installation in which the distance between the sources of radiation is fixed is not capable with good efficiency of processing a great many types of preform made of thermoplastics of different properties and which behave in different ways.
The result of this is that, if several sources need to be installed one after another in order to obtain the required heating power, the preform conveyor needs to have as many deviation means (bends and/or transfer wheels) to divert the path of the preform body on each occasion. Transferring the preforms along a winding path is penalizing on two accounts: firstly, because the conveyor becomes complicated and expensive to produce and secondly because the presence of the bends means that the preforms cannot travel at speeds that are as high as might be desired.